The issue with the second gear input is that the bottom of a couple of the teeth sit very close to the bottom of the internal spline which leaves a very thin cross section in that area which is prone to fracturing that then allows the gear to spread and shatter with bad results for the rest of the gearbox.
It doesn't need the hard shift of an air shifter or the excess load of a car tire launch to split them.
It's common for them to fail on your everyday rider and all it takes is a missed shift or some general hard use for it to go especially with the age and mileage of these bikes and components now.
If i was you i'd buy a fresh billet gear as anything used with an unknown history may be as bad as leaving the stock gear in..
